As is well known, the human eye is sensitive to electromagnetic radiation having a wavelength in a range from about 400 nm to about 700 nm, i.e., in the visible spectrum. Of this range of sensitivity, the human eye is most sensitive to radiation at about 555 nm, which corresponds to the color green. This sensitivity has likely been fine tuned by nature/evolution to match the spectral peak of solar radiation falling on the Earth. Subtle variations in quality of green light strongly affects the quality of white light as perceived by humans.
High-performance blue light-emitting diodes/laser diodes (LEDs/LDs) based on indium-gallium-nitride (InGaN) and red LEDs based on aluminum gallium indium phosphorous (AlGaInP) systems have been available for some time. However, performance of electroluminescent green-light-emitting diodes has been sorely lagging, preventing the realization of white light by color mixing, such as by trichromatic LEDs. This is the so-called “Green Gap.” Various techniques exist in production or are being pursued in research-and-development for realizing a high-performance green laser diode. Examples of such techniques and their drawbacks include: a) frequency doubling (low efficiency); b) using InGaN by modifying crystal orientation of a starting GaN substrate (temperature instability, limited solubility of indium in GaN, high-cost substrate platform, and increased dislocation densities with increased indium content); c) using II-VI wide-band-gap semiconductor materials (short continuous operation limits due to crystal defects); d) using GaInP alloy coatings on GaAs substrate (green LEDs only, no green LDs); e) using NZnO coatings on p-type GaN substrates (broadband green LEDs and no LDs); and f) using quantum dots in lieu of quantum wells (green LEDs only, no green LDs).